Vocational training is a key stage in the educational system for training new professionals. One essential aspect of vocational training curriculums is training in the workplace. In order to improve, we must first know how training in the workplace is developing and discover the success factors in the process that increase its effectiveness. To this aim, we carried out a study entitled «FCT on plus: Avaluació de l'eficàcia de la Formació en Centres de Treball», which was funded by the Fundació Barcelona Formació Professional. Here, we present the qualitative results of this study on success factors that increase the effectiveness of training in the workplace. The data were obtained from a review of scientific literature on the subject and from interviews carried out with stakeholders involved in training in the workplace.